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United States Patent |
5,019,063
|
Marsan
,   et al.
|
May 28, 1991
|
Absorbent articles containing mechanical pulp and polymeric gelling
material
Abstract
A disposable, absorbent diaper comprising a topsheet, a liquid impervious
backsheet associated with the topsheet, and an absorbent core disposed
between the topsheet and the backsheet, wherein the absorbent core has a
dusting layer of mechanical pulp fibers, a continuous primary layer of
mechanical pulp fibers containing from about 8% to about 60%, on a total
primary layer dry weight basis, of discrete particles of polymeric gelling
material, and a water-permeable core reinforcing layer disposed between
the dusting layer and the primary layer. The mechanical pulp fibers,
preferably chemithermomechanical pulp fibers, have an average fiber length
of at least about 1.9 mm and are in a highly disintegrated condition.
Inventors:
|
Marsan; Mario S. (Cincinnati, OH);
Thompson; Leonard R. (Fairfield, OH)
|
Assignee:
|
The Procter & Gamble Company (Cincinnati, OH)
|
Appl. No.:
|
428524 |
Filed:
|
October 30, 1989 |
Current U.S. Class: |
604/368; 604/370; 604/372; 604/375; 604/376; 604/378 |
Intern'l Class: |
A61F 013/15 |
Field of Search: |
604/368,370,372,375,376,378,385.1
|
References Cited
U.S. Patent Documents
3587579 | Oct., 1968 | Sabee | 128/287.
|
3597299 | Aug., 1971 | Thomas et al. | 161/57.
|
3683916 | Aug., 1972 | Mesek et al. | 128/287.
|
3683921 | Aug., 1972 | Brooks et al. | 128/296.
|
3732867 | May., 1973 | Money | 128/290.
|
3768479 | Oct., 1973 | Widlund | 128/287.
|
3867940 | Feb., 1975 | Mesek et al. | 128/287.
|
3927673 | Dec., 1975 | Taylor | 128/287.
|
4141772 | Feb., 1979 | Buell | 156/227.
|
4235237 | Nov., 1980 | Mesek et al. | 128/284.
|
4327729 | May., 1982 | King | 128/287.
|
4392862 | Jul., 1983 | Marsan et al. | 604/366.
|
4673402 | Jun., 1987 | Weisman et al. | 604/368.
|
4765780 | Aug., 1988 | Angstadt | 406/123.
|
4806408 | Feb., 1989 | Pierre et al. | 428/76.
|
4888231 | Dec., 1989 | Angstadt | 604/368.
|
4904440 | Feb., 1990 | Angstadt.
| |
Foreign Patent Documents |
1522569 | Jun., 1976 | GB.
| |
Primary Examiner: Green; Randall L.
Assistant Examiner: Reichle; K. M.
Attorney, Agent or Firm: Lewis; Leonard W.
Claims
What is claimed is:
1. A disposable, absorbent diaper comprising:
(a) a topsheet;
(b) a liquid impervious backsheet associated with said topsheet; and
(c) an absorbent core disposed between said topsheet and said backsheet,
said absorbent core having:
(i) a dusting layer of mechanical pulp fibers;
(ii) a continuous primary layer of mechanical pulp fibers containing from
about 8% to about 60%, on a total primary layer dry weight basis, of
discrete particles of polymeric gelling material; and
(iii) a water-permeable core reinforcing layer disposed between said
dusting layer and said primary layer; wherein: said mechanical pulp fibers
have an average fiber length of at least about 1.9 mm and are
characterized by a Distintegration State of at least about 85%; said
dusting layer has a basis weight of between about 0.01 g/cm.sup.2 and
about 0.20 g/cm.sup.2 and density of between about 0.06 g/cm.sup.3 and
about 0.30 g/cm.sup.3 and said primary layer has a basis weight of between
about 0.01 g/cm.sup.2 and about 0.20 g/cm.sup.2 and a density of between
about 0.06 g/cm.sup.3 and about 0.30 g/cm.sup.3 and said absorbent core is
oriented such that said primary layer is disposed toward said topsheet and
said dusting layer is disposed toward said backsheet.
2. A disposable absorbent diaper, as in claim 1, wherein said diaper is
characterized by the absence of any structural elements interposed between
said topsheet and said primary layer and between said backsheet and said
dusting layer, and a vertical cross section of the absorbent core includes
only one occurrence each of said dusting, primary, and core reinforcing
layers.
3. A disposable absorbent diaper, as in claim 2, wherein the Disintegration
State of the mechanical pulp fibers is at least about 90%.
4. A disposable, absorbent diaper, as in claim 3, wherein said mechanical
pulp fibers are chemithermomechanical pulp fibers which have an average
fiber length of at least about 2.1 mm.
5. A disposable, absorbent diaper, as in claim 4, wherein the
Disintegration State is at least about 95%.
6. A disposable, absorbent diaper, as in claim 4, wherein said dusting
layer is continuous.
7. A disposable, absorbent diaper, as in claim 6, wherein said primary
layer contains from about 12% to about 25% of said polymeric gelling
material.
8. A disposable, absorbent diaper, as in claim 7, wherein said primary
layer contains from about 14% to about 18% of said polymeric gelling
material.
9. A disposable, absorbent diaper, as in claim 8, wherein the primary layer
has an average dry density of from about 0.12 g/cm.sup.3 to about 0.18
g/cm.sup.3 and an average dry basis weight of from about 0.02 g/cm.sup.2
to about 0.12 g/cm.sup.2, and said dusting layer has an average dry
density of from about 0.12 g/cm.sup.3 to about 0.18 g/cm.sup.3 and an
average dry basis weight of from about 0.01 g/cm.sup.2 to about 0.10
g/cm.sup.2, and wherein said absorbent core has an average basis weight of
from about 0.04 g/cm.sup.2 to about 0.15 g/cm.sup.2.
10. A disposable, absorbent diaper, as in claim 9, wherein said absorbent
core has a profiled basis weight with a high basis weight region having a
said absorbent core having a basis weight of about 0.08 g/cm.sup.2 to
about 0.12 g/cm.sup.2 and a low basis weight remainder of said absorbent
core having a basis weight of from about 0.01 g/cm.sup.2 to about 0.06
g/cm.sup.2.
11. A disposable, absorbent diaper, as in claim 7, wherein the primary
layer has an average dry density of from about 0.12 g/cm.sup.3 to about
0.18 g/cm.sup.3 and an average dry basis weight of from about 0.02
g/cm.sup.2, and said dusting layer has an average dry density of from
about 0.12 g/cm.sup.3 to about 0.18 g/cm.sup.3 and an average dry basis
weight of from about 0.01 g/cm.sup.2 to about 0.10 g/cm.sup.2, and wherein
said absorbent core has an average basis weight of from about 0.04
g/cm.sup.2 to about 0.15 g/cm.sup.2.
12. A disposable, absorbent diaper, as in claim 11, wherein said absorbent
core has a profiled basis weight with a high basis weight region having a
said absorbent core having a basis weight of about 0.08 g/cm.sup.2 to
about 0.12 g/cm.sup.2 and a low basis weight remainder of said absorbent
core having a basis weight of from about 0.01 g/cm.sup.2 to about 0.06
g/cm.sup.2.
Description
FIELD OF THE INVENTION
This invention relates to absorbent, disposable articles having an
absorbent core comprising mechanical pulp fibers and discrete particles of
a polymeric gelling material. More particularly, this invention relates to
absorbent, disposable diapers having an absorbent core of mechanical pulp
fibers and polymeric gelling material, wherein the core is internally
reinforced with a core reinforcing element.
BACKGROUND OF THE INVENTION
Absorbent articles, such as disposable diapers, are generally provided with
absorbent cores to receive and retain body fluids. In order for such
absorbent articles to function efficiently, the absorbent cores must
quickly acquire body liquids into the structure from the point of
application and subsequently distribute the body liquids within and
throughout the absorbent core to provide maximum absorbency and
containment. In order to efficiently perform these functions, the
absorbent core should substantially maintain its shape during manufacture,
shipping and packing, and fitting and usage by the user, including usage
subsequent to initial wetting by the user.
Various material have been employed as the absorbent media in disposable
absorbent articles. The most commonly utilized absorbent media in recent
years is chemically pulped wood fibers provided in a highly
individualized, i.e., disintegrated, state referred to in the art as
"fluff". Fluff is typically air-laid in the form of webs, which are then
utilized as the absorbent core of absorbent disposable articles.
Performance of disposable diapers has, in recent years, been improved
through the incorporation into the absorbent cores of polymeric gelling
materials, which have the capability of absorbing fluids to form gels
which lock-away the fluids and which do not release such fluids upon
application of pressure to the absorbent core (e.g., by the user rolling
about after discharging fluids into the absorbent article). Due, at least
in part, to the performance capabilities of disposable absorbent articles
having absorbent cores of chemical pulp fibers and polymeric gelling
materials, the trend has been to manufacture disposable articles having
absorbent cores which are thinner, and of higher density, than
conventionally practiced prior to the widespread usage of polymeric
gelling materials.
Although the chemically pulped fibers commonly used in disposable absorbent
articles provide good overall performance, especially in terms of web
integrity, it is desirable to utilize fibers in disposable absorbent
articles which are more efficient from the standpoint of utilization of
natural resources. Typically, the chemically pulped fibers that are
utilized as fluff in absorbent articles constitute only about 40% of the
wood source from which the fibers are processed. Additionally, it is
desirable to provide absorbent cores of fibers made by processes which
require reduced levels of pulping chemicals (e.g., sulfate in the "Kraft"
chemical pulping process) relative to processes conventionally used to
make chemical pulp fibers. One type of absorbent fibrous media which fits
these objectives is known generally as mechanical pulp, which term
includes such variants as thermomechanical pulp and chemithermomechanical
pulp. The processes used to make these mechanical pulps, which are known
to those skilled in the art, provide substantially higher yields
(typically in excess of 85%) than chemical pulp processes. In addition,
mechanical pulp processes involve reduced usage of processing chemicals.
In spite of the above advantages, mechanically pulped fibers have not been
commonly utilized in commercially marketed disposable absorbent articles,
except in countries where environmental concerns are especially prevalent.
One of the drawbacks conventionally associated with mechanically pulped
fibers is that they provide relatively low strength when formed into
air-laid webs. The use of such low strength webs as absorbent cores in
disposable absorbent articles has conventionally required the use of a
strength-imparting envelope. Webs of low strength mechanical fibers
including an interior reinforcing layer in addition to the required tissue
envelope layers are also known. See, for example, U.S. Pat. No. 4,327,729,
L. W. King, issued May 4, 1982. Unfortunately, the presence of an
additional structural element between the absorbent core and the topsheet
of an absorbent article can impede the rapid absorption of fluids
discharged during use. Other drawbacks traditionally associated with low
strength mechanical pulp webs are that mechanical pulp fibers are less
hydrophilic than chemical pulp fibers and have reduced ability to retain
fluids in the void spaces between fibers. Both of these factors contribute
to reduced performance in the context of absorbent article performance
relative to chemical pulp fibers.
In view of these drawbacks, it is desirable to provide a disposable
absorbent diaper which incorporates, as its primary absorbent fibrous
media, high yield, mechanical pulp fibers. It is an object of the
invention to provide such a disposable absorbent article which has
absorbency and containment performance, as well as absorbent core
structural integrity, comparable to disposable absorbent diapers having
chemically pulped fibers as the principal absorbent fibrous media. It is
particularly an object of this invention to provide disposable absorbent
diapers having thin absorbent cores made from mechanical pulp fibers which
provide absorbency and containment performance, as well as absorbent core
structural integrity, comparable to disposable absorbent articles chemical
pulp fibers and polymeric gelling materials incorporated therein.
These objectives, and other benefits as may become apparent to those
skilled in the art, are believed to be achieved by the invention which is
described hereinafter.
SUMMARY OF THE INVENTION
It has been found that the above objects can be met by disposable absorbent
diapers comprising
(a) a topsheet;
(b) a liquid impervious backsheet associated with said topsheet; and
(c) an absorbent core disposed between said topsheet and said backsheet,
said absorbent core having:
(i) a dusting layer of mechanical pulp fibers;
(ii) a continuous primary layer of mechanical pulp fibers containing from
about 8% to about 60%, on a total primary layer dry weight basis, of
discrete particles of polymeric gelling material; and
(iii) a water-permeable core reinforcing layer disposed between said
dusting layer and said primary layer;
wherein the mechanical pulp fibers have an average fiber length of at least
about 1.9 mm and are characterized by a Disintegration State of at least
about 85%. The dusting layer has an average basis weight of between about
0.01 g/cm.sup.2 and about 0.20 g/cm.sup.2 and a density of between about
0.06 g/cm.sup.3 and about 0.30 g/cm.sup.3. The primary layer has a basis
weight of between about 0.01 g/cm.sup.2 and about 0.20 g/cm.sup.2 and a
density between about 0.06 g/cm.sup.3 and about 0.30 g/cm.sup.3. Also, the
fibrous web is oriented such that said primary layer is disposed toward
the topsheet and the dusting layer is disposed toward the backsheet. The
preferred mechanical pulp fibers are chemically treated or modified
thermomechanical pulp fibers. In an especially preferred embodiment, the
diaper is characterized by the absence of any structural elements
interposed between the topsheet and the primary layer and between said
backsheet and the dusting layer. Additionally, in the preferred
embodiments, the Disintegration State of the fibers is at least about 90%,
more preferably at least about 95%.
While the specification concludes with claims which particularly point out
and distinctly claim the subject matter regarded as constituting the
present invention, it is believed that the invention will be better
understood from the following description taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a disposable diaper embodiment of the present
invention wherein most of the topsheet has been cut-away to more clearly
show the underlying absorbent core of the diaper.
FIG. 2 is a perspective view of the absorbent core of the disposable diaper
of FIG. 1.
FIG. 3 is a perspective view of an alternate embodiment of an absorbent
core that can be utilized in the present invention, wherein the absorbent
core has a profiled basis weight and is shown prior to calendering.
FIG. 4 is a perspective view of a further alternate embodiment of an
absorbent core that can be utilized in the present invention, wherein the
absorbent core has a profiled basis weight and density, and has been
calendered to constant caliper.
FIG. 5 is a cross-sectional view of an absorbent core of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described below in detail in the context of
providing airlaid fibrous webs for use as absorbent cores in disposable
absorbent diapers.
A preferred embodiment of a disposable absorbent diaper is shown, as diaper
20, in FIG. 1. As used herein, the term "diaper" refers to a garment
generally worn by infants and incontinent persons (including adults) that
is worn about the lower torso of the wearer. It should be understood,
however, that the present invention is also applicable to other disposable
absorbent articles corresponding in structure to the diapers described
herein.
FIG. 1 is a plan view of the diaper 20 of the present invention in its
flat-out, uncontracted state (i.e., with all elastic induced contraction
removed) with portions of the structure being cut-away to more clearly
show the construction of the diaper 20 and with the portion of the diaper
20 which contacts the wearer facing the viewer. The diaper 20 is shown in
FIG. 1 to have a front waistband region 22, a back waistband region 24, a
crotch region 26 and a periphery 28 which is defined by the outer edges of
the diaper in which the longitudinal edges are designated 30 and the end
edges are designated 32. The diaper additionally has a transverse
centerline which is designated 34 and a longitudinal centerline which is
designated 36.
The diaper 20 comprises a liquid pervious topsheet 38; a liquid impervious
backsheet 40; an absorbent member 42; and elastic members 44. While the
topsheet 38 the backsheet 40, the absorbent core 42, and the elastic
members 44 may be assembled in a variety of well known configurations, a
preferred diaper configuration is described generally in U.S. Pat. No.
3,860,003 entitled "Contractable Side Portions for Disposable Diaper",
which issued to K. B. Buell on Jan. 14, 1975, and which patent is
incorporated herein by reference.
FIG. 1 shows a preferred embodiment of the diaper 20 in which the topsheet
38 and the backsheet 40 are co-extensive and have length and width
dimensions generally larger than those of the absorbent core 42. The
topsheet 38 is associated with and superimposed on the backsheet 40
thereby forming the periphery 28 of the diaper 20. The periphery 28
defines the outer perimeter or the edges of the diaper 20. The periphery
28 comprises the end edges 32 and longitudinal edges 30.
The diaper 20 has front and back waistband regions 22 and 24 respectively
extending, from the end edges 32 of the diaper periphery 28 toward the
transverse centerline 34 of the diaper a distance from about 2% to about
10%, preferably about 5%, of the length of the diaper 20. The waistband
regions comprise those upper portions of the diaper 20, which when worn,
encircle the waist of the wearer. The crotch region 26 is that portion of
the diaper 20 between the waistband regions 22 and 24, and comprises that
portion of the diaper 20 which, when worn, is positioned between the legs
of the wearer and covers the lower torso of the wearer. Thus, the crotch
region 26 defines the area of typical liquid deposition for a diaper 20 or
other disposable absorbent article.
The topsheet 38 is compliant, soft feeling, and non-irritating to the
wearer's skin. Further, the topsheet 38 is liquid pervious permitting
liquids to readily penetrate through its thickness. A suitable topsheet 38
may be manufactured from a wide range of materials, such as porous foams,
reticulated foams, apertured plastic films, natural fibers (e.g., wood or
cotton fibers), synthetic fibers (e.g., polyester or polypropylene fibers)
or from a combination of natural and synthetic fibers. Preferably, the
topsheet 38 is made of a hydrophobic material to isolate the wearer's skin
from liquids in the absorbent core 42. The topsheet can be treated with a
surfactant in order to facilitate penetration of fluid through said
topsheet, however the topsheet should remain hydrophobic relative to the
fibrous web, shown as absorbent core 42 in FIG. 1.
A preferred topsheet 38 comprises staple length polypropylene fibers having
a denier of about 1.5, such as Hercules type 151 polypropylene marketed by
Hercules, Inc. of Wilmington, Del. As used herein, the term "staple length
fibers" refers to those fibers having a length of at least about 15.9 mm
(0.62 inches).
There are a number of manufacturing techniques which may be used to
manufacture the topsheet 38. For example, the topsheet 38 may be woven,
non-woven, spunbonded, carded, or the like. A preferred topsheet is
carded, and thermally bonded by means well known to those skilled in the
fabrics art. Preferably, the topsheet 38 has a weight from about 18 to
about 25 grams per square meter, a minimum dry tensile strength of at
least about 400 grams per centimeter in the machine direction and a wet
tensile strength of at least about 55 grams per centimeter in the cross
machine direction.
The backsheet 40 is impervious to liquids and is preferably manufactured
from a thin plastic film, although other flexible liquid impervious
materials may be used. The backsheet 40 prevents the exudates absorbed and
contained in the absorbent core 42 from wetting articles which contact the
diaper 20 such as bed sheets and undergarments. Preferably, the backsheet
40 is polyethylene film having a thickness of from about 0.012 mm (0.5
mil) to about 0.051 centimeters (2.0 mils), although other flexible,
liquid impervious materials may be used. As used herein, the term
"flexible" refers to materials which are compliant and which will readily
conform to the general shape and contours of the wearer's body.
A suitable polyethylene film is manufactured by Monsanto Chemical
Corporation and marketed in the trade as Film No. 8020. The backsheet 40
is preferably embossed and/or matte finished to provide a more clothlike
appearance. Further, the backsheet 40 may permit vapors to escape from the
absorbent core 42 while still preventing exudates from passing through the
backsheet 40.
The size of the backsheet 40 is dictated by the size of the absorbent core
42 and the exact diaper design selected. In a preferred embodiment, the
backsheet 40 has a modified hourglass-shape extending beyond the absorbent
core 42 a minimum distance of at least about 1.3 centimeters to about 2.5
centimeters (about 0.5 to about 1.0 inch) around the entire diaper
periphery 28.
The topsheet 38 and the backsheet 40 are associated together in any
suitable manner. As used herein, the term "associated" encompasses
configurations whereby the topsheet 38 is directly joined to the backsheet
40 by affixing the topsheet 38 directly to the backsheet 40, and
configurations whereby the topsheet 38 is indirectly joined to the
backsheet 40 by affixing the topsheet 38 to intermediate members which in
turn are affixed to the backsheet 40. In a preferred embodiment, the
topsheet 38 and the backsheet 40 are affixed directly to each other in the
diaper periphery 28 by attachment means (not shown) such as an adhesive or
any other attachment means as known in the art. For example, a uniform
continuous layer of adhesive, a patterned layer of adhesive, or an array
of separate lines or spots of adhesive may be used to affix the topsheet
38 to the backsheet 40.
Tape tab fasteners 46 are typically applied to the back waistband region 24
of the diaper 20 to provide a fastening means for holding the diaper on
the wearer. Only one of the tape tab fasteners is shown in FIG. 1. The
tape tab fasteners 46 can be any of those well known in the art, such as
the fastening tape disclosed in U.S. Pat. No. 3,848,594, issued to K. B.
Buell on Nov. 19, 1974, which patent is incorporated herein by reference.
These tape tab fasteners 46 or other diaper fastening means, such as pins,
are typically applied near the corners of the diaper 20.
The elastic members 44 are disposed adjacent the periphery 28 of the diaper
20, preferably along each longitudinal edge 30 so that the elastic members
44 tend to draw and hold the diaper 20 against the legs of the wearer.
Alternatively, the elastic members 44 may be disposed adjacent either or
both of the end edges 32 of the diaper 20 to provide a waistband as well
as or rather than leg cuffs. For example, a suitable waistband is
disclosed in U.S. Pat. No. 4,515,595 issued to David J. Kievit and Thomas
F. Osterhage on May 7, 1985, which patent is herein incorporated by
reference. In addition, a method and apparatus suitable for manufacturing
a disposable diaper having elastically contractible elastic members is
described in U.S. Pat. No. 4,081,301 entitled "Method and Apparatus for
Continuously Attaching Discrete, Stretched Elastic Strands to
Predetermined Isolated Portions of Disposable Absorbent Products" which
issued to K. B. Buell on Mar. 28, 1978 and which patent is incorporated
herein by reference.
The elastic members 44 are secured to the diaper 20 in an elastically
contractible condition so that in a normally unrestrained configuration,
the elastic members 44 effectively contract or gather the diaper 20. The
elastic members 44 can be secured in an elastically contractible condition
in at least two ways. For example, the elastic members 44 may be stretched
and secured while the diaper 20 is in an uncontracted condition.
Alternatively, the diaper 20 may be contracted, for example, by pleating,
and the elastic members 44 secured and connected to the diaper 20 while
the elastic members 44 are in their unrelaxed or unstretched condition.
In the embodiment illustrated in FIG. 1, the elastic members 44 extend
essentially the entire length of the diaper 20 in the crotch region 26.
Alternatively the elastic members 44 may extend the entire length of the
diaper 20, or any other length suitable to provide elastically
contractable line. The length of the elastic members 44 is dictated by the
diapers' design
The elastic members 44 may take a multitude of configurations. For example,
the width of the elastic members 44 may be varied from about 25
millimeters (0.01 inches) to about 25 millimeters (1.0 inch) or more; the
elastic members 44 may comprise a single strand of elastic material or may
comprise several parallel or non-parallel strands of elastic material; or
the elastic members 44 may be affixed to the diaper in any of several ways
which are known in the art. For example, the elastic members 44 may be
ultrasonically bonded, heat and pressure sealed into the diaper 20 using a
variety of bonding patterns or the elastic members 44 may simply be glued
to the diaper 20.
The absorbent core 42 is an air-laid fibrous web positioned between the
topsheet 38 and the backsheet 40 to form the diaper 20. The absorbent core
42 is generally compressible, conformable, non-irritating to the wearer's
skin, and capable of absorbing and retaining liquids and certain body
exudates.
FIG. 2 is a perspective view of a preferred embodiment of the absorbent
core 42 of the present invention. The absorbent core 42 is shown in FIG. 2
to comprise a back section 48 and a front section 50 The front section 50
is shown to have an end region 52 and a deposition region 54 Further, the
front section 50 is transversely divided into three regions comprising two
transversely spaced ear regions 60 and 62 respectively, and a central
region 64. The absorbent core 42 additionally has a transverse centerline
which is designated 66 and a longitudinal centerline which is designated
68.
The absorbent core 42 has a back section 48 and a front section 50 that is
contiguous with the back section 48. The back section 48 and the front
section 50 of the absorbent core 42 extend respectively from the end edges
70 of the absorbent core 42 toward the transverse centerline 66, the front
portion 50 extending a distance from about one half to about
three-fourths, preferably about two-thirds, of the length of the absorbent
core 42. The front section 50 is preferably greater than one half of the
total length of the absorbent core 42 so that it will encompass all of the
area of typical liquid deposition of an absorbent core 42 when it is
placed in a diaper or other absorbent article.
The front portion 50 has an end region 52 and a deposition region 54. The
end region 52 comprises that portion of the front section 50 extending
from the respective end edge 70 of the absorbent member 42 toward the
transverse centerline 66 a distance from about 2% to about 10%, preferably
about 5%, of the length of the absorbent core 42. The deposition region 54
comprises that portion of the front portion 50 that is contiguous with and
positioned between the end region 52 and the back section 48 and
encompasses the area of typical liquid deposition of the absorbent core
42.
The front portion 50 further has two transversely spaced ear regions 60 and
62 respectively, and a central region 64 disposed intermediate said ear
regions 60 and 62. The ear regions 60 and 62 comprise those portions which
generally extend from the longitudinal edges 30 of the periphery 28 toward
the longitudinal centerline a distance from about one-tenth to about
one-third of the width of the absorbent core 42. Thus, the ear regions 60
and 62 are those portions that engage the side marginal portions of the
wearer's waist and torso, whereas the central region 64 engages the medial
portion of the wearer's waist and torso. The central region thus defines
the transverse area of typical liquid deposition.
The air-laid fibrous webs of the present invention, such as the absorbent
core 42 of FIG. 2, further comprise a dusting layer 76, having thickness
T.sub.2, and a continuous primary layer 74, having thickness T.sub.1, with
a core reinforcing layer 72 disposed between the dusting layer 76 and the
primary layer 74. "Continuous", as used herein, means that the layer of
the absorbent core which said term references is not interrupted by any
additional reinforcing layers or analogous structural elements disposed
within the interior of said layer, particularly structural elements which
are substantially parallel to the core reinforcing layer between the
primary and dusting layers. The dusting layer is preferably also
continuous.
FIG. 5 shows a cross sectional view of an absorbent core of the present
invention, having primary layer 74, dusting layer 76, and core reinforcing
layer 72 disposed between primary layer 74 and dusting layer 76. Also,
shown are thicknesses T.sub.1 and T.sub.2 in more detail than can be seen
in FIG. 2. Specifically, FIG. 5 shows that thicknesses of the fibrous
absorbent core layers T.sub.1 and T.sub.2, are to be measured exclusive of
the thickness of the core reinforcing layer 72. Other thickness
measurements for fibrous absorbent core layers discussed herein similarly
exclude any core reinforcing layers.
The preferred disposable absorbent diapers of the present invention are
characterized by the absence of the core-enveloping reinforcing elements,
or any other structural elements interposed between the absorbent core and
the topsheet and/or between the absorbent core and the backsheet,
previously utilized for high integrity cores made with primarily
mechanical pulp fibers, sucn as described in U.S. Pat. No. 4,327,729, L.
W. King, "Low-Density Disposable Absorbent Having Low Stretch, Wet
Strength Center Ply to Provide Improved Pad Integrity," issued May 4,
1982, which includes enveloping tissue plies in addition to an internal
core reinforcing layer in the context of low density absorbent cores.
Additionally, the absorbent cores of the diapers of the present invention
have sufficient structural integrity such that they needn't have, and
preferably do not have, areas upon which portions of the core are folded
or layered over itself, such that a vertical cross section of the
absorbent core includes only a single occurrence of the dusting layer, a
single occurrence of the primary layer, and a single occurrence of the
core reinforcing layer. Surprisingly, the absorbent cores of the present
invention exhibit good core integrity while being capable of providing
excellent fluid absorption performance.
While the absorbent cores of the diapers of the present invention do not
have an enveloping tissue layer and do not have portions of the core that
are folded or layered over itself, in one contemplated embodiment of the
invention a liquid pervious sheet is disposed between the topsheet and the
absorbent core, without being connected or otherwise fixably associated
with the topsheet or the absorbent core. Such a sheet which can be made
from the same materials as the topsheet, or other material such as CTMP
tissue, may be provided as a precautionary measure to further inhibit
polymeric gelling material utilized in the absorbent core from contacting
the wearer's skin.
The diapers of the present invention are generally characterized by an
absorbent core of the above construction disposed between a topsheet and
backsheet of a diaper, wherein the fibers utilized in the core are
primarily mechanical pulp fibers. The primary layer and the dusting layer
being continuous, there are no other sheets or other structural elements
incorporated into these core element layers other than the core
reinforcing layer which separates the primary and dusting layers. The
preferred diapers are further characterized by having no other structural
reinforcing elements disposed between the topsheet and backsheet,
respectively, and the absorbent core. Thus, the essential structural
elements of the preferred diaper can be characterized by the absorbent
core, including the interior core reinforcing layer, the continuous
primary layer, including the polymeric gelling material, the continuous
dusting layer and the topsheet and backsheet (which are associated
together as described above). Other features, structures, and elements
directed toward other aspects of diaper performance such as, but not
limited to, fit and leakage features, additional polymeric gelling
material (such as in the dusting layer), etc., are not meant to be
excluded from the scope of the invention.
The core reinforcing layer for use herein are liquid pervious, permitting
liquids to readily penetrate through its thickness. They can be made from
a wide range of materials, including those materials useful for topsheets,
as described above. Substrate materials include porous foams, reticulated
foams, apertured plastic films, tissue sheets from natural fibers (e.g.,
cellulosic fibers such as cotton or wood fibers, including mechanical pulp
fibers such as CTMP), synthetic fibers (e.g., polyester or polypropylene
fibers) or a combination of such natural and/or synthetic fibers. They can
also be treated with material that enables the core reinforcing layer to
maintain a significant portion of their strength subsequent to wetting.
Such treatment, which would include wet strength resins such as are well
known in the art, would be particularly suitable in the case of core
reinforcing layers made from natural cellulosic fibers.
There are a number of manufacturing techniques which may be used to
manufacture the core reinforcing layer, including those techniques that
can be used to manufacture topsheets, as described above. For example, the
core reinforcing layer may be woven, non-woven, spunbonded, carded, or the
like. Preferably, the core reinforcing layer has a weight from about 15 to
about 25 grams per square meter, a minimum dry tensile strength of at
least about 400 grams per centimeter preferably at least about 800 grams
per centimeter in the machine direction and a wet tensile strength of at
least about 55 grams per centimeter in the cross machine direction.
Tensile strength is measured, for purposes herein, according to Tappi
Method T-404 utilizing a Model TM Instron tensile tester, as manufactured
by Instron Engineering Corporation of Canton, Mass., USA.
A preferred core reinforcing layer comprises staple length polypropylene
fibers having a denier of about 1.5, such as Hercules type 151
polypropylene marketed by Hercules, Inc. of Wilmington, Del. This material
is generally made by carding and thermal bonding. An especially preferred
material for the core reinforcing layer is spunbonded polypropylene such
as LUTRASOL (available from Lutrevil Spinvlies, Federal Republic of
Germany) having a basis weight of about 19 g/m.sup.2. Other commercial
materials include: HOLMESTRA (spunbonded polypropylene nonwoven material
available from HJR Fiberweb, Norrkoping, Sweden); PRO-FLEECE (dry laid,
thermally bonded polypropylene nonwoven material available from Amoco
Fabrics, Niederlassung der Amoco Deutschland GmbH, Gronau, F.R.G.); and
LUTRASIL (spunbonded polyamide available from Freudenberg Spunweb, Durham,
N.C., U.S.A.).
The dusting layer 76 is comprised of mechanical pulp fibers. The primary
layer 74 is comprised of mechanical pulp fibers and discrete particles of
polymeric gelling agent.
As used herein, the term "mechanical pulp fibers" shall mean pulp fibers
derived from wood which retain a substantial portion of the lining present
in the unpulped wood. Preferably, greater than about 80% yield based upon
the weight of the unpulped wood will be retained. The mechanical pulp
fibers should have an average fiber length of at least about 1.9 mm,
preferably at least about 2.1 mm, preferably, the mechanical pulp has a
fines content of less than about 15%, as measured according to the Bauer
McNett Screen Test, and a Canadian Standard Freeness (CSF) of at least 200
CSF, preferably at least about 580 CSF. Typically, the CSF will be less
than about 700 for practical reasons, although higher CSF pulp can be
used. Average fiber lengths in conjunction with the description are to be
calculated according to Tappi Standard Method No. T233-05-75 based on data
from a Clark Classifier.
One type of mechanical pulp fibers which can be used is known in the art as
thermomechanical pulp (TMP). TMP pulp fibers are produced by steaming wood
chips at elevated temperature and pressure to soften the lining in the
wood chips. Steaming the wood softens the lining, thereby reducing the
amount of mechanical energy needed for fiberization and consequently
facilitating the production of longer, less damaged fibers and lower
presence of fines than obtainable with more rudimentary mechanical pulp
fibers, such as groundwood, which does not involve a steaming step.
Suitable processes for making TMP are described by Asplund and his
coworkers, as disclosed, for example, in U.S. Pat. No. 2,008,892 (July 23,
1935), incorporated by reference herein.
The highly preferred type of mechanical pulp fibers for use in conjunction
with the present invention are chemithermomechanical pulp (CTMP) fibers,
also sometimes referred to as chemically-modified thermomechanical pulp
fibers. In CTMP processes, wood chips are given a mild chemical treatment
in addition to a heating or steaming step prior to mechanical
defiberization. The chemical treatment, generally a sulfonation or alkali
treatment, is limited so as to merely soften the lignin rather than remove
it as in conventional chemical pulping processes. This chemical treatment
further decreases defiberization energy and allows longer defibrated
fibers to be formed.
A variant of CTMP for which an analogous chemical treatment has been
applied is known as chemimechanical pulp, which omits the steaming step
practiced in the manufacture of TMP and CTMP. It is also known to
chemically treat the pulp after the start or completion of mechanical
defibration. Such treatment can be applied to pulp which has not been
previously chemically treated, or to pulp which has been previously
chemically treated.
Workers such as Beverage and Keough in U.S. Pat. No. 2,422,522 (June 17,
1947), Beverage, Keough and Surino in U.S. Pat. No. 2,425,024 (Aug. 5,
1947) and Asplund, Cederquist and Reinhall in U.S. Pat. No. 3,338,525
(Aug. 29, 1967), all incorporated by reference herein, have generally
described mechanical pulping processes Ford and Gardner, in U.S. Pat. No.
4,116,758 (Sept. 26, 1978), incorporated by reference herein, have
described a mechanical pulping process for producing highly sulfonated
CTMP pulp which also can be of use for the present invention. The
mechanical pulp of this invention will generally be derived primarily from
softwood (gymnosperm) wood sources, although hardwood species can also be
used. Any of the softwood species commonly used for making pulp can be
used. Suitable species include, but are not limited to, Picea glauca
(white spruce), Picea mariana (black spruce), Picea rubra (red spruce),
Pinus strobus (white pine), Pinus caribeau (slash pine), and Pinus tadia
(loblolly pine).
Commercial sources of CTMP pulp include: Cascades (Port Cartier) Inc.
(Quebec, Canada) which markets CTMP pulp from its Port Cartier pulp mill;
Stora Cell AB (Skoghall, Sweden); and Metsa-Serla Group (Tampere,
Finland), which markets CTMP from its Lielahti CTMP mill.
The mechanical pulp fibers used in the fibrous webs should be in an
individualized state referred to in the art as "fluff". The fluff should
have a disintegration state of at least about 85%, preferably at least
about 90%, more preferably at least about 95%. Higher disintegration
states, up to the maximum of 100%, can be used so long as the previously
described fiber length criteria are met. However, such high disintegration
states may be limited in practice in high speed production lines for
practical and processing reasons. The disintegration state, for purposes
herein, shall be determined according to the Disintegration State
Procedure provided below.
In addition to mechanical pulp fibers, the primary layer 74 also comprises
discrete particles of substantially water-insoluble polymeric gelling
material. Polymeric gelling agents are optionally present in the dusting
layer 76, in addition to being essentially present in the primary layer of
the absorbent cores.
Suitable polymeric gelling materials can be inorganic materials such as
silica gels or organic compounds such as cross-linked polymers
Cross-linking may be by covalent, ionic, vander Waals, or hydrogen
bonding. Examples of polymeric gelling materials include polyacrylamides,
polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers,
hydroxypropyl cellulose, carboxymethyl cellulose, polyvinyl morpholinone,
polymers and copolymers of vinyl sulfonic acid, polyacrylates,
polyacrylamides, polyvinyl pyridone and the like. Other suitable Preferred
polymeric gelling materials are those disclosed in Assarason et U.S. Pat.
No. 3,901,236, issued Aug. 26, 1975, the disclosure of which is
incorporated herein by reference. polymeric gelling materials for use in
the absorbent core are hydrolyzed acrylonitrile grafted starch, acrylic
acid grafted starch, polyacrylates, and isobutylene maleic anhydride
copolymers, or mixtures thereof. More preferred are the polyacrylates and
the acrylic acid grafted starch polymeric gelling materials.
Processes for preparing polymeric gelling materials are in Masuda et al.,
U.S. Pat. No. 4,076,663, issued Feb. 28, 1978; in Tsubakimoto et al., U.S.
Pat. No. 4,286,082, issued Aug. 25, 1981; and further in U.S. Pat. Nos.
3,734,876, 3,661,815, 3,670,731, 3,664,343, 3,783,871, and Belgian Patent
No. 785,850, the disclosures of which are incorporated herein by
reference.
Particularly preferred polymeric gelling materials are the polyacrylates
described in U.S. Pat No. 32,649 issued to K. A. Brandt et al. on Apr. 19,
1988, which is incorporated herein by reference.
The preferred polymeric gelling materials, referred to as hydrogel forming
material in U.S. Pat. No. 32,649, are characterized generally by
consisting essentially of (a) from about 50 mole percent to 99.999 mole
percent of polymerized unsaturated, polymerizable; and (b) from about
0.001 mole percent to 5 mole percent of a cross-linking agent; wherein the
composition has a degree of neutralization of at least about 25% and is
substantially free of graft polymerizable polymer moieties; and further
wherein said polymer composition, upon neutralization to a degree of
neutralization of at least 50%, has or would have a gel volume of at least
about 20 grams of synthetic urine per gram of hydrogel-forming polymer, a
gel strength such that the hydrogel formed from said polymer exhibits a
shear modulus of at least about 3270 dynes/cm.sup.2, an initial
extractable polymer content, after one hour in synthetic urine, of no more
than about 7.5% by weight of hydrogel-forming polymer, and an equilibrium
extractable polymer content, at equilibrium in synthetic urine, of no more
than about 17% by weight of hydrogel-forming polymer, such descriptive
limits being determined according to the techniques disclosed in U.S. Pat.
No. 32,649.
The polymeric gelling agents used in the primary layer herein, and
optionally in the dusting layer, are in the form of discrete particles.
Polymeric gelling agents can be of any shape, e.g., spherical or
semi-spherical, cubic, rod-like, polyhedral, etc. Shapes having a large
greatest dimension/smallest dimension ratio, like needles, flakes and
fibers, are also contemplated for use herein. Conglomerates of polymeric
gelling agents particles may also be used.
Although the polymeric gelling agent webs herein are expected to perform
well with particles having a particle size varying over a wide range,
other considerations may preclude the use of very small or very large
particles. For reasons of industrial hygiene, average particle sizes
smaller than about 30 microns are less desirable. Particles having a
smallest dimension larger than about 2 mm may also cause a feeling of
grittiness in the absorbent article, which is undesirable from a consumer
aesthetics standpoint. Furthermore, rate of fluid absorption can be
affected by polymeric gelling agents particle size. Larger particles
typically have very much reduced rates of absorption. Preferred for use
herein are polymeric gelling agent particles having an average particle
size of from about 50 microns to about 1 mm. "Particle Size" as used
herein means the weighted average of the smallest dimension of the
individual particles.
The relative amount of polymeric gelling agent fiber material and polymeric
gelling agent particles used in the primary layer of the absorbent cores
of the articles herein can be most conveniently expressed in terms of a
weight percentage (dry weight basis) of the primary layer. The primary
layer of the absorbent cores herein contain from about 8% to 60%,
preferably from about 12% to 25%, more preferably from about 14% to about
18%, most preferably about 16%, by weight of the primary layer of
polymeric gelling agents.
The density and basis weight of the polymeric gelling agent containing
primary layer of the absorbent core can be of some importance in
determining the absorbent properties of the resulting absorbent article.
The average dry density of the primary layer should be in the range of
from about 0.06 to about 0.30 g/cm.sup.3, and more preferably within the
range of from about 0.12 to about 0.18 g/cm.sup.3. Typically the average
dry basis weight of the primary layer is from about 0.01 to 0.20
g/cm.sup.2, preferably from about 0.02 to about 0.12 g/cm.sup.2. The same
ranges of density can be utilized for the dusting layer, however average
basis weight of the dusting layer preferably is between about 0.01
g/cm.sup.2 and about 0.12 g/cm.sup.2, preferably about 0.01 g/cm.sup.2 to
about 0.10 g/cm.sup.2. Also preferably, the dusting layer will have a
thickness (i.e., caliper) which is preferably from about 1/3 to about 3/4,
preferably about 2/5 of the thickness of the primary layer. Also
preferably, the absorbent core will have an average basis weight of
between 0.04 g/cm.sup.2 and about 0.15 g/cm.sup.2. Typically, the primary
layer and dusting layer will have approximately the same density for
absorbent cores not having profiled density or basis weight, as discussed
in more detail below. Density values for the primary and dusting layers
are calculated from basis weight and layer caliper measured on newly
unpacked, unfolded and dissected articles Caliper is measured according to
Tappi Standard Method T-411. Density and basis weight values include the
weight of the polymeric gelling agent particles.
In a preferred embodiment of the present invention, the primary layer will
comprise an intimate admixture of mechanical pulp fiber material and
polymeric gelling agent particles with the polymeric gelling agent
particles being substantially uniformly distributed throughout a
hydrophilic fiber matrix. Fibrous web layers of this type can be formed by
air-laying a dry mixture of the fibers and polymeric gelling agent
particles and densifying the resulting web. Such a procedure is described
more fully in Procter & Gamble; European Patent Publication No. EP-A-No.
122,042; Published Oct. 17, 1984, incorporated herein by reference. As
indicated in this reference, the webs formed by this procedure for use as
the primary layer will preferably comprise substantially unbonded fibers
and will preferably have a moisture content of 10% or less.
The particles of polymeric gelling agent may be dispersed in various weight
ratios throughout different regions and thicknesses of the primary layer
or may be homogeneously dispersed throughout the entire primary layer. For
example, the mixture of mechanical pulp fibers and particles of polymeric
gelling agents may be disposed only in the deposition region 54 of the
absorbent core 42 and not in the back section 48 or the end region 52.
The airlaid webs hereof can be made by processes known in the art for
making two layer webs having an interior reinforcing layer such as
described by Kenneth B. Buell in U.S. Pat. No. 4,141,772, issued Feb. 27,
1979, incorporated herein by reference. A preferred process for making the
airlaid webs is described generally by John J. Angstadt in U.S. Pat. No.
4,765,780, issued Aug. 23, 1988, incorporated herein by refence. U.S. Pat.
No. 4,765,780 discloses a process and apparatus for forming airlaid
fibrous webs having a multiplicity of components, such as a two layer
absorbent core with one layer having polymeric gelling agent homogenously
blended therein and the other layer being substantially free of polymeric
gelling agent. These two layers are formed using a single drum, by feeding
the fluff to two separate chutes, with the fluff directed toward one of
the chutes having polymeric gelling material added thereto. The fluff from
each of the chutes is sequentially deposited onto the drum to provide a
two layer web. In order to make the airlaid webs of the present invention
with a core reinforcing layer, such as a nonwoven polypropylene material,
utilizing the apparatus of the general type described in U.S. Pat. No.
4,765,780, the core reinforcement layer can be deposited upon the first
layer of fluff deposited on the drum through the use of feed rolls
positioned between the two fluff chutes such that the core reinforcement
layer is sequentially deposited between the deposition of the two fluff
layers. Air baffles, should be used to shield the feed rolls and core
reinforcement layer material from air turbulence created by rotation of
the drum. This can be done by routing the core reinforcement layer from
the feed rolls through the orifice of an additional, conventional chute,
such that said core reinforcement layer is deposited at a position
sequentially between the chutes depositing the two fluff layers at an
intermediate position on the drum between where the primary layer and the
dusting layer are deposited making the present absorbent cores, else the
core reinforcing layer would interfere with subsequent deposition of
fibrous material.
The absorbent cores of the present invention preferably have density and
basis weight profile to provide additional fibrous material in the region
of bodily fluid discharge, such that the regions of bodily fluid discharge
are characterized by a higher degree of capillarity relative to the rest
of the absorbent core.
As used herein, the "region of fluid discharge" of the airlaid web
corresponds to the proximate area of discharges made by the wearer of the
diaper. With respect to FIGS. 2 and 4, this refers to the regions common
with both the central regions 64 and 464 and the front regions 54 and 454,
respectively. The portion of the absorbent core having increased density
and basis weight can, of course, extend beyond the region of fluid
discharge and also needn't encompass all of such discharge region.
However, the said portion should be coextensive with at least part, and
preferably all, of the fluid discharge region of an absorbent core.
The region of the diaper with increased density and basis weight may vary
for diaper designs depending upon whether the diaper is intended for use
by primarily males, primarily females, or both.
FIGS. 3 and 4 show alternate embodiments of absorbent cores of the present
invention that are especially designed for disposable diaper use whereby
particularized regions of the absorbent cores have increased basis weight,
alternately referred to herein as profiled basis weight.
FIG. 3 shows one alternative embodiment of an absorbent core 342 of the
present invention which has a forward-profiled basis weight. The basis
weight is profiled to provide a higher basis weight in the region of
discharge by the user relative to the low based weight remainder of the
absorbent core. Absorbent core 342 has primary 374, dusting layer 376,
core reinforcing layer 372, a high basis weight front section 350, back
section 348, and basis weight transition zone 378. As shown in FIG. 3, the
front section 350 of the absorbent member 342 has a thickness T3, greater
than the thickness T4, of the back section 348. The front section 350 of
the absorbent core 342 has a terraced character by virtue of the thickness
difference between the front section 350 and the back section 348 and by
virtue of the relatively steep slope formed by a basis weight transition
zone which is designated 378. Such a core can be made by airlaying a
thickness profiled core-preform of substantially uniform density.
Preferably, for uncalendered absorbent cores, T3 is at least about 1.5
times as great as T4 and preferably about 2.0 times as great as T4.
Preferably, the absorbent core 342 is calendered, most preferably to
approximately uniform caliper, to provide a front section 350 which is of
about 1.5 times, preferably about 2.0 times, higher basis weight and
density than the back section 348. The type of basis weight profile shown
in FIG. 3 is particularly useful for diapers worn by male users. Preferred
density and basis weight for the front section 350 are from about 0.12
g/cm.sup.3 to about 0.30 g/cm.sup.3, more preferably from about 0.12
g/cm.sup.3 to about 0.18 g/cm.sup.3, and from about 0.04 g/cm.sup.2 to
about 0.15 g/cm.sup.2, most preferably from 0.08 g/cm.sup.2 to 0.12
g/cm.sup.2, respectively. The low basis weight remainder of the absorbent
core preferably has an average basis weight of between about 0.01
g/cm.sup.2 and about 0.06 g/cm.sup.2 ; more preferably about 0.03 to about
0.05 g/cm.sup.2.
FIG. 4 shows an embodiment of an absorbent core 442, having an alternate
profiled basis weight, subsequent to calendering to uniform caliper.
Absorbent core 442 has a back section 448, a front section 450 with an end
region 452, a deposition region 454, ear regions 460 and 462, and central
region 464, transverse centerline 466, longitudinal centerline 468, end
edges 470, core reinforcing layer 472, primary layer 474, dusting layer
476, and a densified, high basis weight region 482. Absorbent core 442 has
an elliptically-shaped densified region 482 which encompasses regions of
typical discharge for male and female users. Such design is particularly
useful for female users since the high basis weight region 482 is located,
in use, primarily in proximity to the crotch of the wearer, where the
discharges of female wearers are concentrated.
DISINTEGRATION STATE PROCEDURE
Three samples of dry disintegrated mechanical pulp fibers of approximately
0.75 g-1.25 g each are prepared by preconditioning state determination
maintained at 23.0.degree. C. .+-.1.0.degree. C. and at 50% .+-.5%
relative humidity for sufficient time for said fibers to equilibrate (8-12
hours of equilibration is typically sufficient). Each sample is weighed
and any clumps of non-disintegrated fibers are removed. A clump, for
purposes herein is any grouping of five or more fibers bonded together,
typically by hydrogen bonding. A pair of hand-held tweezers are then used
to hold and shake each clump, for the purpose of removing loose,
non-bonded fibers. The clumps from each of the three samples of fibers are
then weighed. The original masses of the three fiber samples are added
together to determine the total fiber mass of the fiber samples. The
masses of the clumps of nondisintegrated fibers are added together to
determine the total nondisintegrated fiber mass. The Disintegration State
is calculated as follows:
##EQU1##
While particular embodiments of the present invention have been illustrated
and described, it would be obvious to those skilled in the art that
various changes and modifications can be made without departing from the
spirit and scope of the invention. The invention is to be defined
according to the following claims, which are intended to encompass such
changes and modifications.
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